1. Field of the Invention
The present invention relates to a liquid crystal display device. More particularly, the present invention relates to a technology, which is effective when applied to a liquid crystal display device with a backlight unit of a light guide plate system (also referred to as an edge light type or a side light type).
2. Description of the Related Art
Conventionally, a liquid crystal display device with a backlight unit of a light guide plate system is used as, for example, a display portion of portable information equipment such as a cellular telephone terminal.
There is a tendency for portable information equipment to handle, for example, more massive image information and video information which includes more frames as communication speed becomes higher and the capacity of an embedded memory is increased. In association with this, medium-sized and small-sized liquid crystal display devices used as a display portion of the portable information equipment have higher image quality and a larger screen, and further, tend to have more pixels. With regard to the number of the pixels, there is a transition to increase from a conventional quarter video graphics array (QVGA) of 240×320×3 pixels to a video graphics array (VGA) of 480×620×3 pixels.
Further, medium-sized and small-sized liquid crystal display devices used as the display portion of the portable information equipment is required to have, for example, longer battery life and higher brightness for improving outdoor visibility. Because the increase in the number of the pixels in the liquid crystal display devices ordinarily results in a decreased aperture ratio, especially with regard to a VGA liquid crystal display device, it is difficult to make the battery life longer and improve the brightness. Therefore, in recent years, to improve transmission efficiency of the liquid crystal display device is again considered.
The display principle of a liquid crystal display device is summarized as follows. A liquid crystal display device mainly includes a liquid crystal display panel and a backlight disposed at the back thereof. Here, the liquid crystal display panel includes a pair of substrates, a liquid crystal layer sandwiched between the pair of substrates, and a pair of polarizing plates disposed so as to sandwich the liquid crystal layer therebetween. Further, here, ordinarily, the pair of polarizing plates are disposed so as to sandwich the pair of substrates and the liquid crystal layer therebetween. A display region of the liquid crystal display panel includes a large number of pixels as described above, and each of the pixels includes a pixel electrode, a common electrode, and the liquid crystal layer.
Each of the pixels of the liquid crystal display panel produces black by blocking light from the backlight, and produces white by transmitting light from the backlight. The pair of polarizing plates are often disposed such that the absorption axes thereof are orthogonal to each other. In this case, if the polarization state of light which has passed through the polarizing plate on the backlight side is not changed by the liquid crystal layer, the light is completely absorbed by the other polarizing plate to produce black. Further, in this case, if the polarization state of light which has passed through the polarizing plate on the backlight side is changed by the liquid crystal layer, the light passes through the other polarizing plate to produce white.
Light from a backlight used in a liquid crystal display device is partially polarized light which is close to natural light, and hence ordinarily, almost 60% of the light is absorbed by the polarizing plate. This is one of the main reasons that the transmission efficiency is reduced in the liquid crystal display device.
In the conventional liquid crystal display device, in order to decrease absorption by a polarizing plate of light from a backlight, a reflective polarizing plate, for example, is used. The reflective polarizing plate is, for example, laminated on a surface of the polarizing plate which is in proximity to the backlight, and reflects a component of the light from the backlight which is absorbed by the polarizing plate to the backlight side. The light reflected by the reflective polarizing plate is reflected again by the light guide plate, a prism sheet, and the like of the backlight, and enters the reflective polarizing plate again. In the course of this process, if the light which enters the reflective polarizing plate again has a transmission component, the transmission component passes through the reflective polarizing plate and may be used for the display. If all the light reflected by the reflective polarizing plate enters the reflective polarizing plate again, and the ratio of the transmission component to the entire re-incident light is 100%, the light use efficiency must be approximately doubled. However, in a liquid crystal display device having the above-mentioned structure, the light use efficiency by using the reflective polarizing plate is increased only up to 1.3 times as the light use efficiency without using the reflective polarizing plate.
In order to still improve the light use efficiency, in a recent backlight including a light guide plate, a reflective polarizing plate is disposed in proximity to a light source as disclosed in, for example, Japanese Patent Application Laid-open No. 2003-007114.